To date studies on the remineralization of enamel lesions have been carried out by us on artificial lesions. Although the lesions were identical with those of enamel caries long exposure times were required. The recent development of a "single section" technique by us whereby caries-like lesions are created in single sections has the following important advantages: 1) short exposure times; 2) continual monitoring of lesions; 3) each lesion acts as its own control in experiments on progression and remineralization; 4) the sound enamel can be studied prior to producing a lesion over the same site. Thus, much greater precision in the monitoring of lesions is possible using this technique. In addition, the development of a lesion micro-dissection technique by us, coupled with high resolution, SEM, has made it possible to examine single crystals in specific zones of the lesion. This has already provided the first direct evidence of crystal growth as a result of remineralization. The proposal deals with seven specific areas of study. 1) A comparison of the susceptibility of sound enamel, remineralized sound enamel, and acid-etched and remineralized sound enamel surfaces to lesion initiation in vitro. 2) The effect of altering the calcium and phosphate levels in the calcifying fluid; four different calcium levels with phosphate constant, and four different phosphate levels with calcium remaining constant will be studied. 3) The effects of single and multiple exposure periods of enamel lesions to the calcifying fluids. A series of six-minute exposure periods will be evaluated as well as six one-minute exposures using the single section technique. 4) The effect of very low levels of fluoride in the calcifying fluids, from 1 - 0.001 ppm, will be studied. 5) The effect of various remineralization regimens on lesion progression. Since the single section technique will be used, each lesion will act as its own control over a series of progressions. 6) Lesion initiation progression and remineralization will be investigated using another new model system where pairs of whole teeth are set up in interproximal contact in silicone blocks. Lesions are created at the contact regions in each tooth. Using this technique it will be possible to correlate the macroscopic, histological and bitewing radiographic appearances of artificial lesions in a clinical-like situation. 7) Further work will be carried out on the effect of in vitro pellicle on the initiation, progression and remineralization of enamel lesions.